With the increase of the recording density of magnetic recording, the surface property of the magnetic layer becomes more smooth, whereby the problem that the coefficient of friction between the magnetic layer and each part in a recording and reproducing apparatus during running is increased, and the obstruction of the smooth running of the magnetic recording medium or damage of the magnetic layer becomes remarkable.
Furthermore, with the popularization of video tape recorders (VTR), personal computers, word processors, etc., the conditions, particularly the temperature and humidity conditions in use of magnetic recording media recently become wide. Also, magnetic recording media are used for the system of repeating the on-off action of a magnetic head onto the magnetic layer many times in the form of rotary magnetic media such as floppy disks for personal computers and word processors, the aforesaid problem of the running durability of a magnetic recording medium under high temperature conditions or the condition that a thermocycle from a high temperature to a low temperature is continued for a long period of time becomes an important issue.
For solving the aforesaid problems, in a so-called coating type magnetic recording medium when the magnetic layer is mainly composed of a ferromagnetic powder and a binder resin, a method of adding a fatty acid ester into the magnetic layer (as described in JP-A-50-22603, JP-A-50-153905, and JP-A-55-139637 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-B 39-28367, JP-B-41-18065, and JP-B-47-12950 (the term "JP-B" as used herein means an "examined Japanese patent publication")), a method of adding a silicone compound into the magnetic layer (as described in U.S. Pat. No. 2,654,681), a method of adding a fatty acid or a hydrocarbon into the magnetic layer, etc., are proposed.
However, in such conventional techniques, there are problems that the aforesaid additives are liable to vaporize from the surface of the magnetic layer at a high temperature and on the other hand, the additives deposit on the surface of the magnetic layer at a low temperature to cause clogging of a magnetic head or increasing of drop out, etc. Also, when a large amount of the additive is added to the magnetic layer for obtaining the increased effect, the additive plasticizes the binder resin of the magnetic layer to weaken the film strength of the magnetic layer, whereby the durability is reduced.
For solving the aforesaid problems, a method of adding a fatty acid ester having a high molecular weight and having a branched hydrocarbon group or an unsaturated hydrocarbon group to the magnetic layer so that the additive does not deposit on the surface of the magnetic layer at a low temperature is disclosed in JP-A-58-160425, JP-A-58-218038, JP-A-60-205827, JP-A-61-294637, and JP-A-62-125529.
However, since these esters are in liquid state at normal temperature, are easily compatible with a binder resin, and plasticize the binder resin, which results in reducing the film strength of the magnetic layer, the sufficient effect is not obtained by the use of such esters. On the other hand, the ester compounds having a straight chain alkyl group are relatively excellent in lubricating property but many of them have a high melting point, and hence the use of such ester compounds frequently cause a problem that the additive deposits on the surface of the magnetic layer at low temperatures.
The aforesaid problems in the conventional techniques become severer as the surface of the magnetic recording medium becomes smoother. For example, in a so-called coating type magnetic recording medium having a magnetic layer mainly composed of a ferromagnetic powder and a binder resin wherein a polar group-containing polymer having a large capability of accelerating the dispersion of the ferromagnetic powder is used as the binder resin (as disclosed in JP-A-57-133521, JP-A-57-44227, JP-A-60-238309, and JP-A-61-104329) or in a so-called metal thin layer type magnetic recording medium having a thin layer of a ferromagnetic metal or alloy as the magnetic layer wherein the surface roughness of the magnetic layer is less than 0.01 .mu.m, the aforesaid problems become more serious.
Accordingly, an acceptable magnetic recording medium having a magnetic layer having a smooth surface and being suitable for high density recording has not yet been developed with respect to running durability under expanded environmental conditions.